1. Field of the Invention
Boron nitride coatings on ceramic fibers are useful in ceramic matrix composites. Boron nitride provides an intermediate phase with low shear strength, between the fiber and the matrix at which cracks propagating through the matrix are deflected. The crack deflection ability of the boron nitride coating, and its resistance to oxidation and reaction with various ceramics, provides a mechanism for toughening the composite and for preventing serious mechanical failures. Boron nitride is typically applied to fibers by chemical vapor deposition (CVD). Although CVD produces good quality coatings, the process requires long application times, complex gas handling equipment and, thus, is not economical.
To avoid the disadvantages experienced with the current CVD methods of boron nitride application, various alternatives to CVD applied boron nitride have been investigated. Most involve the use of "pre-ceramic" polymers containing boron and nitrogen and the steps of synthesizing these polymers, applying these polymers to substrates, evaporating the solvent and pyrolyzing the coatings to obtain boron nitride. Some of the polymers are based on borazine. Others use boron hydrides. These processes are cumbersome as they require a multiple step synthesis of the polymer and may leave a carbon residue from the polymer employed. Further, the carbon residue may react with the fibers or the matrix and weaken the composite or may be oxidized easily.
The current invention avoids the problems of both CVD processes and alternative procedures using "pre-ceramic" polymers, by first applying a solution containing a boron hydride ion directly to a substrate then converting the coating on the substrate to boron nitride by pyrolysis in an ammonia atmosphere. Application of boron hydride ion salts to substrates offers an advantage over pre-ceramic polymers because of their simpler preparation and use. In addition, the present process offers a high conversion of boron salts to boron nitride and eliminates the problems associated with the handling of toxic volatile boron compounds.
2. Prior Art
Various processes involving the synthesis and use of boron nitride or boron nitride with boron carbide have been reported.
Rees, William Smith, Jr. and Dietmar Seyferth, J. Am. Ceram. Soc., 71(4)C-194-C-196 (1988) disclose B.sub.10 H.sub.12 diamine polymers that can be processed to serve as BN precursors. Their pyrolysis at 1000.degree. C., under an atmosphere of NH.sub.3 yields BN.
U.S. Pat. No. 4,642,271 which claims a ceramic fiber composite containing boron nitride.
Toshinobu Yogo, Shigeru Matsuo, Shigeharu Naka, Yogyo Kyokaishi, 95(1), 94-8 (1987) disclose the synthesis of boron nitride from triammonia decaborane and ammonia at 800.degree. C. and atmospheric pressure.
None of the references uncovered by applicant's search involve the application of solutions of B.sub.10 H.sub.10.sup.-2 and B.sub.12 H.sub.12.sup.-2 salts along with a soluble polymer as taught herein.